Halogens can displace less reactive halogens from their aqueous salt solutions. Which of the following halogen displacement reactions will occur spontaneously as written?

Introduction / Context:
Halogens are reactive non metals that can undergo displacement reactions in aqueous solution. A more reactive halogen can displace a less reactive halogen from its halide salt solution. This concept is used to illustrate reactivity trends in group 17 of the periodic table. The question presents several possible reactions and asks which one will occur spontaneously, meaning the more reactive halogen is displacing the less reactive one from its compound.

Given Data / Assumptions:
- The halogens considered are fluorine, chlorine, bromine and iodine, with a known reactivity order.
- Reactivity of halogens decreases down the group: fluorine is most reactive, followed by chlorine, bromine and then iodine.
- A displacement reaction is spontaneous if the free halogen on the left side is more reactive than the halide ion of the halogen in solution.
- All reactions are considered in aqueous solution under standard conditions for these simple comparisons.

Concept / Approach:
The key rule for these reactions is that a halogen can displace another halogen that is below it in the group from the corresponding halide salt. Thus, chlorine can displace bromine and iodine from their salts, while bromine can displace iodine but not chlorine or fluorine. Fluorine, being most reactive, can in principle displace all others, whereas iodine, being least reactive, cannot displace any of the others. To decide if a reaction is spontaneous, we check whether the halogen element on the left is more reactive than the halogen in the halide on the right.

Step-by-Step Solution:
Step 1: Recall the reactivity order for halogens: F2 > Cl2 > Br2 > I2. Step 2: Examine option A: Cl2 reacting with fluoride ions to produce F2. Chlorine is less reactive than fluorine, so it cannot displace fluorine from fluoride; this reaction is not spontaneous. Step 3: Examine option B: I2 reacting with fluoride ions to produce F2. Iodine is much less reactive than fluorine and cannot displace fluorine from fluoride; this reaction is also not spontaneous. Step 4: Examine option C: Cl2 reacting with bromide ions to produce Br2. Chlorine is more reactive than bromine (higher in the group), so it can displace bromine from bromide; this reaction is spontaneous. Step 5: Examine options D and E, which involve iodine or bromine trying to produce more reactive halogens from their halide ions. In each case, a less reactive halogen is trying to displace a more reactive one, so these reactions will not proceed spontaneously. Therefore, option C is the only spontaneous reaction as written.

Verification / Alternative check:
Laboratory experiments confirm these trends. When chlorine gas is bubbled through a solution of sodium bromide, the solution turns brown due to the formation of bromine. This visually demonstrates that chlorine displaces bromine from bromide. However, when bromine is added to a chloride solution, no displacement of chlorine occurs, and similarly iodine does not displace bromine or chlorine. Fluorine is so reactive that it is not commonly used in simple classroom experiments, but the trend is well established from thermodynamic data. These observations support the conclusion that the reaction of chlorine with bromide ions is spontaneous, as shown in option C.

Why Other Options Are Wrong:
In option A, chlorine cannot oxidise fluoride ions to fluorine because fluorine is already more reactive and has a stronger tendency to gain electrons. Option B is even less favourable, as iodine is the least reactive halogen and cannot displace fluorine. Option D involves iodine trying to displace bromine, which is slightly more reactive, so the reaction does not proceed in that direction. Option E suggests that bromine can displace fluorine, which contradicts the known reactivity order. All these cases show a less reactive halogen attempting to displace a more reactive one, which is not spontaneous under normal conditions.

Common Pitfalls:
Students may confuse reactivity trends of halogens with those of metals, where reactivity increases down a group in some cases. Another common mistake is to look only at the products and assume that any formation of a halogen element on the right side is possible without checking relative reactivities. To avoid such errors, learners should memorise or recall the simple descending order of halogen reactivity and apply the rule that a more reactive halogen can displace a less reactive halogen from solution, but not the other way around.

Final Answer:
The halogen displacement reaction that occurs spontaneously is Cl2(g) + 2NaBr(aq) --> Br2(l) + 2NaCl(aq).